Will you explain how you added the sound to the work space? I'd like to try this myself but have found no help or instruction from the discord or reddit or anywhere.
Sorry for slow reply, I'm not very active on here.
I just learned this myself so there are probably more efficient ways than mine, but mine is better than nothing =).
First off, I imagine many sounds are not part of the base game, Assuming you're using the Better Repack version of KK, go to your [utility] kkmanager folder, run the manager, look for mod updates, dl them (takes forever for your first time).
After that, I presume your UI is similar to mine. So in Chara studio, goto the 'add' menu, and select 'items'. alot at the top are female voice 'item's, but the ones I'm talking about is titled 3dsfx or 3dsound or something like that. Add the sound you want to use/try for the first time. Then in the workspace menu, where you click those checkboxes to show/hide stuff in your scene, look for the recently added sound. click on it, and then open your timeline with control-t if you haven't yet.
This part is kinda un-intuitive. while your sound is highlighted, on the left-side of the timeline where you usually see the things like 'camera rotation, camera zoom', etc. Look for one that says 'enable' (I usually have to scroll down a few pages to find it). middle-click on 'enable', err not the word itself, but some position on the time bar thing, but on the same line as 'enable'. You should probably notice everything moving down a line. That's because (for me at least), the first time you click on THAT enable (middle mouse click, yes your scroll wheel probably), suddenly there is a NEW 'enable' timeline thing, generally near the top of the timeline menu thing. This one is the one you actually use to activate your sounds when you want on the timeline. While your thing is checked/activated in the workspace thing, middle-click on the new enable whenever you want the sound to activate.
Many sounds have a duration much longer than t heir actual sound though, and if you tell it to 'enable' when it didn't fully complete a prior activation (or if it's checked to begin with), it wont do anything. So unfortunately, you have to middle-click shortly after every time you activate t he sound and set it to disabled in order to make future enables fully work. It's probably a good idea to middle-click right at the start of the scene at 0:00:00 and make a 'disable' there too just to make sure your first one plays properly in case you accidentally forget to uncheck your sound in the workspace menu checklist thing.